This an unmissable story related achievement. Once you get to mission 5, just follow the waypoints however you see fit. Eventually, you'll come across two female cultists talking. Deal with them however you see fit, and then you'll climb a white rock wall just past where they were. At the top, you'll go into a small cave and see a portion of a giant human face made of white rock with a glowing red stone eye. Interact with the eye and the achievement will pop.